This invention relates to fluorescent lamps, and, in particular, to fluorescent lamp configurations which are self-ballasting.
Conventional fluorescent lamps exhibit what is known as a negative volt-ampere characteristic. That is to say, after discharge arc initiation between the lamp electrodes, increased current through the lamp results in lower lamp impedance and a corresponding drop in voltage across the lamp. An effect of this lowered impedance is the tendency of the current in the lamp to increase to undesirable levels. To prevent this from occurring, conventional fluorescent lamp circuits employ a ballasting device or circuit to provide current limitation. Such ballasts are often in the form of relatively heavy and costly magnetic coils with high magnetic permeability core material. The presence of a relatively large inductor in the lamp circuit acts to limit the lamp current. However, the negative volt-ampere characteristic of the lamp requires such ballasting circuits and, accordingly, conventional fluorescent lamps must generally be provided in long luminaire structures with separate ballasting devices. Accordingly, because of the required lamp structures, fluorescent lamps are generally incompatible with conventional incandescent lamp sockets and fixtures.
While it is true that fluorescent lamps exhibit a negative volt-ampere characteristic, it is also well known that electrical discharges in capillary tubing exhibit a positive volt-ampere characteristic, even for low pressure gas fills of approximately 1 torr. The reason for this characteristic is not completely understood although it is felt that it is due to electron-ion momentum scattering. One consequence of such a positive volt-ampere characteristic is that a discharge in such a capillary tube may be operated directly from a firm voltage source without a ballast. However, lamps consisting of capillary tubing only are impractical for several reasons. First, such lamps are hard to restart each half cycle because of rapid cleanup of discharge products at the alternating supply current passes through its zero value. Second, even though the mercury radiation from such lamps is relatively bright to the eye, the overall lumen efficacy under these conditions is also known to be low. Third, the current density in these lamps is also high since the cross-sectional area is low. In fact, the current density approaches 100 amperes per square centimeter. For these reasons, there is considerable segregation of the mercury by thermal and cataphoretic effects. Additionally, mercury radiation along the length of the capillary tubing is nonuniform. After a few minutes, the mercury is driven to the ends of the tube because this is the cooler region and the discharge reverts to a rare gas only discharge. Thus, while capillary tubing discharges are known to possess a positive volt-ampere characteristic and are also capable of directly operating from a conventional alternating current power line, such capillary structures do not, by themselves, provide good lamps.